Dichlorophenols are useful intermediates in the preparation of a variety of chemical products. In particular, certain herbicides are prepared from dichlorophenols. For example, 2,4-dichlorophenoxyacetic acid (2,4-D) can be prepared from 2,4-dichlorophenol. See, for example, U.S. Pat. Nos. 2,480,817 and 2,651,659. Also, 3,6-dichloro-2-methoxybenzoic acid (dicamba) can be prepared from 2,5-dichlorophenol. See, for example, U.S. Pat. Nos. 3,013,054 and 5,648,562.
With the introduction of dicamba and 2,4-D tolerant traits in various crop plants, these herbicides are becoming increasingly important. Difficulties remain in processes for efficiently producing dicamba. High raw material cost, low process conversions and selectivities, and large amounts of wastes are problems that remain in these processes. In addition, the 2,5-dichlorophenol starting material is expected to significantly contribute to high raw material costs. Accordingly, there remains a need for improved processes in the production and separation of critical intermediates to dicamba, including 2,5-dichlorophenol, to improve process economics.
Processes for producing 2,4-D typically use 2,4-dichlorophenol as a starting material. However, processes used to prepare 2,4-dichlorophenol from phenol can produce mixtures of dichlorophenols. Accordingly, there remains a need for improved processes for separating 2,4-dichlorophenol from other dichlorophenols such as 2,6-dichlorophenol and/or 2,5-dichlorophenol to provide a relatively pure stream of 2,4-dichlorophenol suitable for the production of 2,4-D.
Mixtures of mono- and dichlorophenols, especially those containing 2,4-dichlorophenol and 2,5-dichlorophenol, are exceedingly difficult to separate by conventional distillation techniques since the boiling points of these compounds are within no more than few degrees centigrade of each other. U.S. Pat. No. 3,772,394 proposes an extraction process for selectively extracting with an aqueous alkaline solution a component of a mixture of 2,3-dichorophenol and 2,6-dichorophenol or a mixture of 2,5-, 2,4- and 3,4-isomers of dichorophenol. In this process, an extract is obtained in which the proportion of at least one of the dichlorophenols to another of the dichlorophenols is greater than in the feed mixture. However, this process fails to achieve significantly enriched fractions for more than one component of the feed mixture thereby requiring additional separation operations to recover more than one purified fraction. Thus, there remains a need for cost effective separation processes that can produce enriched fractions of multiple components of a chlorophenol feed mixture such as 2,4-dichlorophenol and 2,5-dichlorophenol.